U.S. patent number 8,665,206 [Application Number 13/206,367] was granted by the patent office on 2014-03-04 for driving method to neutralize grey level shift for electrophoretic displays.
This patent grant is currently assigned to SiPix Imaging, Inc.. The grantee listed for this patent is Jiing Shiuh Chu, Craig Lin. Invention is credited to Jiing Shiuh Chu, Craig Lin.
United States Patent |
8,665,206 |
Lin , et al. |
March 4, 2014 |
Driving method to neutralize grey level shift for electrophoretic
displays
Abstract
The present invention provides driving methods for a display
having a binary color system of a first color and a second color,
which methods can effectively neutralize the grey level shifts due
to degradation of a display medium.
Inventors: |
Lin; Craig (San Jose, CA),
Chu; Jiing Shiuh (Kaohsiung, TW) |
Applicant: |
Name |
City |
State |
Country |
Type |
Lin; Craig
Chu; Jiing Shiuh |
San Jose
Kaohsiung |
CA
N/A |
US
TW |
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Assignee: |
SiPix Imaging, Inc. (Fremont,
CA)
|
Family
ID: |
45564524 |
Appl.
No.: |
13/206,367 |
Filed: |
August 9, 2011 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20120038687 A1 |
Feb 16, 2012 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61372418 |
Aug 10, 2010 |
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Current U.S.
Class: |
345/107 |
Current CPC
Class: |
G09G
3/344 (20130101); G09G 3/2059 (20130101); G09G
2320/043 (20130101) |
Current International
Class: |
G09G
3/34 (20060101) |
Field of
Search: |
;345/107 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Kao, W.C., Ye, J.A. and Lin, C. (Jan. 2009) Image Quality
Improvement for Electrophoretic Displays by Combining Contrast
Enhancement and Halftoning Techniques. ICCE 2009 Digest of
Technical Papers, 11.2-2. cited by applicant .
Kao, W.C., Ye, J.A., Chu, M.I. and Su, C.Y. (Feb. 2009) Image
Quality Improvement for Electrophoretic Displays by Combining
Contrast Enhancement and Halftoning Techniques. IEEE Transactions
on Consumer Electronics, 2009, vol. 55, Issue 1, pp. 15-19. cited
by applicant.
|
Primary Examiner: Nguyen; Chanh
Assistant Examiner: Pham; Long D
Attorney, Agent or Firm: Perkins Coie LLP
Parent Case Text
BENEFIT CLAIM
This application claims the benefit, under 35 U.S.C. 119(e), of
prior provisional application 61/372,418, filed Aug. 10, 2010, the
entire contents of which are hereby incorporated by reference for
all purposes as if fully set forth herein.
Claims
What is claimed is:
1. A driving method for an electrophoretic display, which
comprises: a) selecting a first waveform or a second waveform to
drive a pixel to a desired color, wherein said first waveform
shifts intermediate color states between a first color state and a
second color state towards the first color state after degradation,
and said second waveform shifts the intermediate color states
between the first color state and the second color state towards
the second color state after degradation; b) determining a shift
error value from a grey level variation chart based on the selected
first or second waveform in (a) above and the desired color of the
pixel; c) adding the shift error value to a cumulative error value
of said pixel; and d) performing error diffusion based on the
cumulative error value and the grey level variation chart.
2. The method of claim 1, wherein said step (a) is carried out
based on the cumulative error value of the pixel.
3. The method of claim 2, wherein the first waveform is selected if
the cumulative error value indicates the shift to the second color
state after degradation or the second waveform is selected if the
cumulative error value indicates the shift to the first color state
after degradation.
4. The method of claim 1 wherein step (d) comprises: i) diffusing
the sum of the shift error value and the cumulative error value of
the pixel, to the neighboring pixels; and ii) adding the error
value diffused to the cumulative error value resulted from
processing of previous pixels, for each neighboring pixel.
5. The method of claim 1, wherein said cumulative error values for
each pixel are generated in a waveform map.
6. The method of claim 1, wherein step (a) is carried out by: i)
determining shift error values for both the first waveform and the
second waveform from the grey level variation chart based on the
desired color of a pixel, wherein said first waveform shifts the
intermediate color states between the first color state and the
second color state towards the first color state after degradation,
and said second waveform shifts the intermediate color states
between the first color state and the second color state towards
the second color state after degradation; ii) adding each of the
shift error values to the cumulative error value of the pixel; and
iii) selecting the first waveform or the second waveform whose sum
of the shift error value and the cumulative error value has a
smaller absolute value.
7. The method of claim 6, wherein said step (d) comprises: i)
diffusing the sum of the shift error value and the cumulative error
value of said pixel, to neighboring pixels; and ii) adding the
error diffused to the cumulative error value resulted from
processing of previous pixels, for each neighboring pixel.
8. The method of claim 6, wherein the cumulative error values for
each pixel are generated in a waveform map.
9. The method of claim 1, wherein said first waveform and said
second waveform are white to grey and black to grey waveforms,
respectively.
10. The method of claim 1, wherein said first waveform and said
second waveform are white to black to grey and black to white to
grey waveforms, respectively.
11. The method of claim 1, wherein said first waveform and said
second waveform are mono-polar waveforms.
12. The method of claim 1, wherein said first waveform said the
second waveform are bi-polar waveforms.
Description
FIELD OF THE INVENTION
The present invention relates generally to electrophoretic
displays.
BACKGROUND OF THE INVENTION
An electrophoretic display is a device based on the electrophoresis
phenomenon of charged pigment particles dispersed in a solvent. The
display usually comprises two electrode plates placed opposite of
each other and a display medium comprising charged pigment
particles dispersed in a solvent is sandwiched between the two
electrode plates. When a voltage difference is imposed between the
two electrode plates, the charged pigment particles may migrate to
one side or the other, depending on the polarity of the voltage
difference, to cause either the color of the pigment particles or
the color of the solvent to be seen from the viewing side of the
display.
Factors which may negatively affect the performance of an
electrophoretic display include optical response speed decay of the
display and the grey level shift under operating conditions. The
decay in performance is often due to photo-exposure, temperature
variation and aging of the materials used in the display
device.
SUMMARY OF THE INVENTION
The present invention is directed to a driving method, which
comprises:
a) selecting a first waveform or a second waveform to drive a pixel
to a desired color, wherein said first waveform tends to shift the
intermediate color states between the first and second colors
states towards the first color after degradation, and said second
waveform tends to shift the intermediate color states between the
first and second color states towards the second color after
degradation;
b) determining a shift error value from a grey level variation
chart based on the waveform selected in (a) above and the desired
color of the pixel;
c) adding the shift error value to the cumulative error value of
said pixel; and
d) performing error diffusion.
In one embodiment, step (a) is carried out based on the cumulative
error value of the pixel. The first waveform is selected if the
cumulative error value indicates a shift to the second color after
degradation or the second waveform is selected if the cumulative
error value indicates a shift to the first color after
degradation.
In another embodiment, step (a) is carried out by:
i) determining shift error values for both a first waveform and a
second waveform from a grey level variation chart based on the
desired color of a pixel, wherein said first waveform tends to
shift the intermediate color states between the first and second
colors states towards the first color after degradation, and said
second waveform tends to shift the intermediate color states
between the first and second color states towards the second color
after degradation;
ii) adding each of the shift error values to the cumulative error
value of the pixel; and
iii) selecting the first waveform or the second waveform whose sum
of the shift error value and the cumulative error value has a
smaller absolute value.
Step (d) of the driving method comprises:
i) diffusing the sum of the shift error value and the cumulative
error value of the pixel, to the neighboring pixels; and
ii) adding the error value diffused to the cumulative error value
resulted from processing of previous pixels, for each neighboring
pixel.
The cumulative error values for a pixel in the display device are
generated in a waveform map.
The driving methods of the present invention can effectively
neutralize the grey level shifts due to degradation of a display
medium.
BRIEF DISCUSSION OF THE DRAWINGS
FIG. 1 illustrates an electrophoretic display.
FIGS. 2a-2c show an example of a binary color system.
FIG. 3 shows an example of mono-polar waveforms suitable for the
driving methods of the present invention.
FIG. 4 is a graph which shows how the response speed degrades after
time.
FIG. 5 shows another example of mono-polar waveforms.
FIGS. 6a and 6b show examples of bi-polar waveforms suitable for
the driving methods of the present invention.
FIG. 7 is a block diagram of hardware for Example 3.
FIG. 8 is a block diagram of hardware for Example 4.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates an electrophoretic display (100) which may be
driven by the driving methods presented herein. In FIG. 1, the
electrophoretic display cells 10a, 10b, 10c, on the front viewing
side indicated with a graphic eye, are provided with a common
electrode 11 (which is usually transparent and therefore on the
viewing side). On the opposing side (i.e., the rear side) of the
electrophoretic display cells 10a, 10b and 10c, a substrate (12)
includes discrete pixel electrodes 12a, 12b and 12c, respectively.
Each of the pixel electrodes 12a, 12b and 12c defines an individual
pixel of the electrophoretic display. However, in practice, a
plurality of display cells (as a pixel) may be associated with one
discrete pixel electrode.
It is also noted that the display device may be viewed from the
rear side when the substrate 12 and the pixel electrodes are
transparent.
An electrophoretic fluid 13 is filled in each of the
electrophoretic display cells. Each of the electrophoretic display
cells is surrounded by display cell walls 14.
The movement of the charged particles in a display cell is
determined by the voltage potential difference applied to the
common electrode and the pixel electrode associated with the
display cell in which the charged particles are filled.
As an example, the charged particles 15 may be positively charged
so that they will be drawn to a pixel electrode or the common
electrode, whichever is at an opposite voltage potential from that
of charged particles. If the same polarity is applied to the pixel
electrode and the common electrode in a display cell, the
positively charged pigment particles will then be drawn to the
electrode which has a lower voltage potential.
In another embodiment, the charged pigment particles 15 may be
negatively charged.
In a further embodiment, the electrophoretic display fluid could
also have a transparent or lightly colored solvent or solvent
mixture and charged particles of two different colors carrying
opposite particle charges, and/or having differing electro-kinetic
properties. For example, there may be white pigment particles which
are positively charged and black pigment particles which are
negatively charged and the two types of pigment particles are
dispersed in a clear solvent or solvent mixture.
The charged particles 15 may be white. Also, as would be apparent
to a person having ordinary skill in the art, the charged particles
may be dark in color and are dispersed in an electrophoretic fluid
13 that is light in color to provide sufficient contrast to be
visually discernable.
The term "display cell" is intended to refer to a micro-container
which is individually filled with a display fluid. Examples of
"display cell" include, but are not limited to, microcups,
microcapsules, micro-channels, other partition-typed display cells
and equivalents thereof. In the microcup type, the electrophoretic
display cells 10a, 10b, 10c may be sealed with a top sealing layer.
There may also be an adhesive layer between the electrophoretic
display cells 10a, 10b, 10c and the common electrode 11.
In this application, the term "driving voltage" is used to refer to
the voltage potential difference experienced by the charged
particles in the area of a pixel. The driving voltage is the
potential difference between the voltage applied to the common
electrode and the voltage applied to the pixel electrode. As an
example, in a binary system, positively charged white particles are
dispersed in a black solvent. When no voltage is applied to a
common electrode and a voltage of +15V is applied to a pixel
electrode, the "driving voltage" for the charged pigment particles
in the area of the pixel would be +15V. In this case, the driving
voltage would move the positively charged white particles to be
near or at the common electrode and as a result, the white color is
seen through the common electrode (i.e., the viewing side).
Alternatively, when no voltage is applied to a common electrode and
a voltage of -15V is applied to a pixel electrode, the driving
voltage in this case would be -15V and under such -15V driving
voltage, the positively charged white particles would move to be at
or near the pixel electrode, causing the color of the solvent
(black) to be seen at the viewing side.
The term "binary color system" refers to a color system has two
extreme color states (i.e., the first color and the second color)
and a series of intermediate color states between the two extreme
color states.
FIG. 2 is an example of a binary color system in which white
particles are dispersed in a black-colored solvent.
In FIG. 2A, while the white particles are at the viewing side, the
white color is seen.
In FIG. 2B, while the white particles are at the bottom of the
display cell, the black color is seen.
In FIG. 2C, the white particles are scattered between the top and
bottom of the display cell; an intermediate color is seen. In
practice, the particles may spread throughout the depth of the cell
or are distributed with some at the top and some at the bottom. In
this example, the color seen would be grey (i.e., an intermediate
color).
While black and white colors are used in the application for
illustration purpose, it is noted that the two colors can be any
colors as long as they show sufficient visual contrast. Therefore
the two colors in a binary color system may also be referred to as
a first color and a second color.
The intermediate color is a color between the first and second
colors. The intermediate color has different degrees of intensity,
on a scale between two extremes, i.e., the first and second colors.
Using the grey color as an example, it may have a grey scale of 8,
16, 64, 256 or more. In a grey scale of 8, grey level 0 may be the
full black color and grey level 7 may be the full white color. Grey
levels 1-6 are grey colors ranging from dark to light.
The present inventors have now found driving methods for a display
having a binary color system of a first color and a second color,
which methods can effectively neutralize the grey level shifts due
to degradation of a display medium.
Before discussing the specifics of the driving methods, the error
diffusion technique which is an essential feature of the methods is
briefly described in the following.
Error diffusion is generally known to be a type of halftoning or
spatial dithering in which the residual error is distributed to
neighboring pixels which have not yet been processed. The error
diffusion process may be a one dimensional or two dimensional error
diffusion process. The one dimensional error diffusion technique is
the simplest form of the algorithm and scans the image one row at a
time and one pixel at a time. The error is then added to the value
of the next pixel in the image and the process repeats. The
algorithm of the two dimensional error diffusion is exactly like
one dimensional error diffusion, except, for example, half the
error is added to the next pixel and one quarter of the error is
added to the pixel on the next line below and one quarter of the
error is added to the pixel on the next line below and one pixel
forward.
Floyd-Steinberg dithering is another error diffusion technique
commonly used by image manipulation processor. The algorithm
achieves dithering by diffusing the residual error of a pixel to
its neighboring pixels, according to the distribution:
.function. ##EQU00001##
where "--" denotes a pixel in the current row which has already
been processed (hence diffusing an error to it is not possible),
and "#" denotes the pixel currently being processed.
The algorithm scans the image from left to right, top to bottom,
processing pixel values one by one. Each time the residual error is
transferred to the neighboring pixels, while not affecting the
pixels that already have been processed. Hence, if a number of
pixels have been rounded downwards, it becomes more likely that the
next pixel is rounded upwards, such that on average, the error is
normalized to be close to zero.
Another method is referred to as "minimized average error," and
uses a larger kernel:
.function. ##EQU00002##
The present invention is directed to a driving method for a display
having a binary color system of a first color and a second color,
which comprises:
a) selecting a first waveform or a second waveform to drive a pixel
to a desired color, wherein said first waveform tends to shift the
intermediate color states between the first and second colors
states towards the first color after degradation, and said second
waveform tends to shift the intermediate color states between the
first and second color states towards the second color after
degradation;
b) determining a shift error value from a grey level variation
chart based on the waveform selected in (a) above and the desired
color of the pixel;
c) adding the shift error value to the cumulative error value of
the pixel; and
d) performing error diffusion.
In a first aspect of the present invention, the selection step (a)
is carried out based on the cumulative error value for the pixel,
resulted from processing of previous pixels. In addition, if the
cumulative error value indicates a shift to the second color after
degradation, the first waveform would be selected, and if the
cumulative error value indicates a shift to the first color after
degradation, the second waveform would be selected.
In the method described above, the term "a desired color" is
intended to refer to the first color, the second color or an
intermediate color of any level.
One pixel at a time is processed for error diffusion. Therefore the
term "cumulative" error for a pixel is intended to refer to the
error value accumulated from processing of previous pixels.
The shift error value in step (b) is determined from a grey level
variation chart. The shift error value is the difference between
the intended grey level and the actual grey level displayed. The
grey level variation chart is unique to each display device because
the chart may vary from one display device to another display
device, depending on the medium property of each display device. In
the grey level variation chart, the variation for each grey level
expressed in a grey scale of a higher order is preferred. For
example, while a display device may display images in a grey scale
of 16 levels (e.g., 0-15), in the operation of error diffusion, the
variation of each grey level is preferably expanded to a grey scale
of 256. This step is necessary for the sake of precision, because
the variation for each grey level may only be expressed in the form
of an integer. A specific example of a grey level variation chart
is given below.
The error diffusion step (d) may comprise:
i) diffusing the sum of the shift error value and the cumulative
error value of the pixel, to the neighboring pixels; and
ii) adding the error value diffused to the cumulative error value
resulted from processing of previous pixels, for each neighboring
pixel.
In performing the error diffusion, in the context of the present
invention, a waveform map is used, in which the cumulative error
value due to grey level shift for each pixel is indicated. Based on
the cumulative error value, an appropriate waveform is selected for
each pixel, as discussed above for step (a) of the method.
The second aspect of the present invention is directed to an
alternative driving method. In this aspect, the selection of the
waveform is carried out in a different manner, which comprises the
following steps:
i) determining shift error values for both a first waveform and a
second waveform from a grey level variation chart based on the
desired color of a pixel, wherein said first waveform tends to
shift the intermediate color states between the first and second
colors states towards the first color after degradation, and said
second waveform tends to shift the intermediate color states
between the first and second color states towards the second color
after degradation;
ii) adding each of the shift error values to the cumulative error
value of the pixel; and then
iii) selecting the first waveform or the second waveform whose sum
of the shift error value and the cumulative error value has a
smaller absolute value.
The error diffusion step of this alternative method is the same as
that for the first aspect of the invention, which may comprise:
i) diffusing the sum of the shift error value and the cumulative
error value of the pixel, to neighboring pixels; and
ii) adding the error diffused to the cumulative error value
resulted from processing of previous pixels, for each neighboring
pixel.
The present driving methods are suitable for not only display
devices with a degraded medium but also for those with a fresh
medium. When the methods are carried out on a display device with a
fresh medium, the exact steps of error diffusion as described
herein will be followed. As a result, the display driving system
does not need to know the state of the medium degradation when
carrying out the present methods and good image quality can be
achieved in both cases.
More details are demonstrated in the following examples.
EXAMPLES
Example 1
Mono-Polar Waveforms
FIG. 3 shows an example of the first and second waveforms referred
to in the methods as described. As shown, the two waveforms marked
as "WG" and "KG" waveforms have three driving phases (I, II and
III). Each driving phase has a driving time of equal length, T,
which is sufficiently long to drive a pixel to a full white or a
full black state, regardless of the previous color state.
For illustration purpose, FIG. 3 represents an electrophoretic
fluid comprising positively charged white pigment particles
dispersed in a black solvent.
The common electrode is applied a voltage of -V, +V and -V during
Phase I, II and III, respectively.
For the WG waveform, during Phase I, the common electrode is
applied a voltage of -V and the pixel electrode is applied a
voltage of +V, resulting a driving voltage of +2V and as a result,
the positively charged white pigment particles move to be near or
at the common electrode, causing the pixel to be seen in a white
color. During Phase II, a voltage of +V is applied to the common
electrode and a voltage of -V is applied to the pixel electrode for
a driving time duration of t1. If the time duration t1 is 0, the
pixel would remain in the white state. If the time duration t1 is
T, the pixel would be driven to the full black state. If the time
duration t1 is between 0 and T, the pixel would be in a grey state
and the longer t1 is, the darker the grey color. After t1 in Phase
II and also in Phase III, the driving voltage for the pixel is
shown to be 0V and as a result, the color of the pixel would remain
in the same color state as that at the end of t1 (i.e., white,
black or grey). Therefore, the WG waveform is capable of driving a
pixel to a full white (W) color state (in Phase I) and then to a
black (K), white (W) or grey (G) state (in Phase II).
For the KG waveform, in Phase I, both the common and pixel
electrodes are applied a voltage of -V, resulting in 0V driving
voltage and as a result, the pixel remains in its initial color
state. During Phase II, the common electrode is applied a voltage
of +V while the pixel electrode is applied a voltage of -V,
resulting in a -2V driving voltage, which drives the pixel to the
black state. In Phase III, the common electrode is applied a
voltage of -V and the pixel electrode is applied a voltage of +V
for a driving time duration of t2. If the time duration t2 is 0,
the pixel would remain in the black state. If the time duration t2
is T, the pixel would be driven to the full white state. If the
time duration t2 is between 0 and T, the pixel would be in a grey
state and the longer t1 is, the lighter the grey color. After t2 in
Phase III, the driving voltage is 0V, thus allowing the pixel to
remain in the same color state as that at the end of t2. Therefore,
the KG waveform is capable of driving a pixel to a full black (K)
state (in Phase II) and then to a black (K), white (W) or grey (G)
state (in Phase III).
The term "full white" or "full black" state is intended to refer to
a state where the white or black color has the highest intensity
possible of that color for a particular display device. Likewise, a
"full first color" or a "full second color" refers to a first or
second color state at its highest color intensity possible.
Either one of the two waveforms (WG and KG) can be used to generate
a grey level image as long as the lengths of the grey pulses are
correctly chosen for the grey levels to be generated.
It is noted that varying durations of t1 or t2 in the WG and KG
waveforms provide different levels of the grey color. However, in
practice, t1 or t2 is fixed in the WG and KG waveforms to achieve a
particular grey level. But as the response speed becomes slower due
to environmental conditions or aging of the display device, the
fixed t1 or t2 in the waveforms would drive the display device to a
grey level which is not the same as the originally intended grey
level.
FIG. 4 is a graph which shows how the response speed degrades after
time. In the figure, for the WG waveform, line WG(i) is the initial
curve of reflectance at different grey levels (0-15), and line
WG(d) is the curve of reflectance at different grey levels (0-15)
after degradation of the display medium. For the KG waveform, line
KG(i) is the initial curve of reflectance at grey different levels
(0-15) and line KG(d) is the curve after degradation.
As shown, the grey levels show a higher reflectance when driven by
the WG waveform due to medium degradation. In other words, the grey
levels achieved by the WG waveform tend to shift towards the white
color state. As a result, the colors of the images driven by the
degraded WG waveform would appear washed out.
On the other hand, the grey levels show a lower reflectance when
driven by the KG waveform due to medium degradation. In other
words, the grey levels achieved by the KG waveform tend to shift
towards the black color state. As a result, the colors of the
images driven by the degraded KG waveform would appear darker.
In addition, as shown in FIG. 4, the degree of shift between WG(i)
and WG(d) is not the same as the degree of shift between KG(i) and
KG(d). For example, the reflectance of grey level 4 has shifted
from 9.6% to 19.6% with the WG waveform and the reflectance of grey
level 4 has shifted from 9.8% to 4.9%, with the KG waveform. In
other words, the WG waveform has shifted +10% (becoming lighter) in
reflectance while the KG waveform has shifted -4.9% (becoming
darker) in reflectance.
When waveforms WG and KG are used, one of the methods of the
present invention may be summarized as follows:
a) selecting the WG or KG waveform to drive a pixel to a desired
color, based on a cumulative error value resulted from processing
of previous pixels, wherein the WG waveform tends to shift the grey
level color states between the black and white colors states
towards the white color after degradation, and the KG waveform
tends to shift the grey level color states between the black and
white color states towards the black color after degradation;
b) determining a shift error value from a grey level variation
chart based on the waveform selected in (a) above and the desired
color of the pixel;
c) adding the shift error value to the cumulative error value of
the pixel; and
d) performing error diffusion.
The alternative driving method may be summarized as follows:
a) determining shift error values for both the WG and KG waveforms
from a grey level variation chart based on the desired color of a
pixel, wherein the WG waveform tends to shift the grey level color
states between the black and white colors states towards the white
color after degradation, and the KG waveform tends to shift the
grey level color states between the black and white color states
towards the black color after degradation;
b) adding each of the shift error values to the cumulative error
value of the pixel;
c) selecting the WG waveform or the KG waveform whose sum of the
shift error value and the cumulative error value has a smaller
absolute value;
d) determining a shift error value from a grey level variation
chart based on the waveform selected in (c) above and the desired
color of the pixel;
e) adding the shift error value to the cumulative error value of
the pixel; and
f) performing error diffusion.
Example 2
A Grey Level Variation Chart
TABLE-US-00001 Intended WG Waveform KG Waveform Grey Initial
Degraded Initial Degraded Level Actual Actual Actual Actual 0 0 0 0
0 1 40 82 41 0 2 51 109 63 20 3 69 150 87 26 4 118 197 120 25 5 145
210 134 42 6 166 218 158 58 7 174 220 171 72 8 187 230 182 101 9
194 232 197 112 10 208 234 210 140 11 220 236 215 151 12 225 236
224 177 13 232 238 229 188 14 235 238 235 207 15 255 255 255
255
In this example, grey level 0 indicates a full black state and grey
level 15 indicates a full white state. When expressed in a grey
scale of 256 levels, similarly, level 0 indicates a full black
state and level 255 indicates a full white state.
The chart also shows that there may be a slight variation in the
initial state between the WG and the KG waveforms, when expanded to
a higher order. For example, for the intended grey level 5, the WG
waveform shows an initial state of 145 while the KG waveform shows
an initial state of 134, expressed in a grey scale of 256. This is
due to driving limitation of the platform (e.g., frame time); but
this can be improved if the system is operated in a higher
frequency.
The chart also shows how speed decay affects the grey levels. For
the WG waveform, the grey level variation tends to trend higher (a
positive variation) which indicates that the grey levels displayed
after degradation are brighter than originally intended. For the KG
waveform, the grey level variation tends to trend lower (a negative
variation) which means that the grey levels displayed after
degradation are darker than originally intended. This phenomenon in
fact is essential for selecting an appropriate waveform (WG or KG)
for a particular pixel in order to neutralize the reflectance
increase or decrease due to speed decay.
Example 3
Error Diffusion and Waveform Map
In this example, a display image of 12 pixels (A-L) is used to
illustrate error diffusion.
TABLE-US-00002 A B C D E F G H I J K L
The target image in this example is:
TABLE-US-00003 A(10) B(5) C(4) D(7) E(5) F(4) G(8) H(7) I(5) J(4)
K(5) L(5)
This means that in the target image, the 12 pixels A-L are driven
to grey levels 10, 5, 4, 7, 5, 4, 8, 7, 5, 4, 5 and 5
respectively.
The following is a sequence of waveform maps showing how the method
is carried out: Starting Waveform Map:
TABLE-US-00004 A(0) B(0) C(0) D(0) E(0) F(0) G(0) H(0) I(0) J(0)
K(0) L(0)
Waveform Map after Pixel A is processed:
TABLE-US-00005 A(WG) B(+11) C(0) D(0) E(0) F(0) G(+8) H(+2) I(0)
J(0) K(0) L(0)
Waveform Map after Pixel B is processed:
TABLE-US-00006 A(WG) B(KG) C(-35) D(0) E(0) F(0) G(-7) H(-23) I(-5)
J(0) K(0) L(0)
Waveform Map after Pixel C is processed:
TABLE-US-00007 A(WG) B(KG) C(WG) D(+19) E(0) F(0) G(-7) H(-15)
I(+9) J(+3) K(0) L(0)
The Starting Waveform Map is the initial state of the waveform map
in which each pixel shows a cumulative error of 0.
As the error diffusion progresses in the waveform map from left to
right and top to bottom, the process is performed from pixel A to
pixel L, one pixel at a time.
For pixel A, since the cumulative error is 0, either waveform WG or
waveform KG may be chosen. If waveform WG is selected, the shift
error value based on the grey level variation chart in Example 2
would be +26 (234-208) for grey level 10 (which is the target grey
level for pixel A).
Then based on the Floyd-Steinberg algorithm, this error of +26 is
diffused to the neighboring pixels: +11 (+26.times. 7/16) to pixel
B, +8 (+26.times. 5/16) to pixel G and +2 (+26.times. 1/16) to
pixel H, as shown in the Waveform Map, after Pixel A is
processed.
For pixel B, it has already shown a positive cumulative error of
+11 in Waveform Map after Pixel A is processed. As indicated above,
a positive cumulative error value is indicative of a pixel the grey
level of which tends to shift to a lighter color. Therefore
waveform KG is selected to neutralize the shift.
The target grey level of pixel B is 5. According to the grey level
variation chart for waveform KG in Example 2, a shift error value
of -92 (42-134) would occur for grey level 5. This shift error
value of -92 is then mathematically added to the existing
cumulative error value (from processing of previous pixels) of +11
for pixel B, resulting in a cumulative error value of -81. The
cumulative error of -81 is then diffused to the neighboring pixels
(C, G, H & I) based on the Floyd-Steinberg algorithm. The
result is shown in the Waveform Map, after Pixel B is
processed.
It is noted that the error value diffused from pixel B must be
mathematically added to the existing cumulative error value
resulted from processing of previous pixels. For example, pixel G
already has a cumulative error value at this stage of +8 and now an
error value of -15 (-81.times. 3/16) is diffused to this pixel,
resulting in a cumulative error of
-7 in the Waveform Map, after Pixel B is processed.
For pixel C, it has already shown a negative cumulative error of
-35. Therefore waveform WG is selected to neutralize the shift to a
darker color.
The target grey level of pixel C is 4. According to the grey level
variation chart for waveform WG in Example 2, a shift error value
of +79 (197-118) would occur for grey level 4. This shift error
value of +79 is then mathematically added to the existing
cumulative error value of -35 for pixel C, resulting in a
cumulative error value of +44. The cumulative error of +44 is then
diffused to the neighboring pixels (D, H, I & J) based on the
Floyd-Steinberg algorithm. The result is shown in the Waveform Map,
after Pixel C is processed.
This process continues (from left to right and top to bottom) until
the waveform map is complete to show which pixel is driven by which
waveform. Final Waveform Map:
TABLE-US-00008 A(WG) B(KG) C(WG) D(KG) E(WG) F(KG) G(WG) H(KG)
I(WG) J(WG) K(KG) L(WG)
The method as demonstrated may reduce the errors (caused by speed
degradation) to substantially zero.
It is noted that while the Floyd-Steinberg algorithm is used in
this example, other error diffusion algorithms may be similarly
applied.
Example 4
Block Diagram of Hardware for Example 3
A block diagram in FIG. 7 illustrates the method demonstrated in
Example 3. As shown, based on the cumulative error value for a
pixel in waveform map (70), a waveform (either the first waveform
71a or the second waveform 71b) is selected. Both the selected
waveform and the desired color (72) of the pixel then are input
into the look-up table module (73). The data thus generated from
the look-up table module are output to the display panel.
In the meantime, the sum of the shift error value from a grey level
variation chart (74) based on the selected waveform and desired
color (72), and the cumulative error for the pixel in the waveform
map (70) undergoes the process of error diffusion (75). The error
value diffused to each of the neighboring pixels is then
mathematically added to the cumulative error value for that
neighboring pixel, resulting in an updated waveform map. The
process as described continues.
Example 5
Alternative Driving Method
In this example demonstrating an alternative driving method, the
display image of 12 pixels (A-L) as shown in Example 3 and the same
target image are used for illustration purpose:
TABLE-US-00009 A B C D E F G H I J K L A(10) B(5) C(4) D(7) E(5)
F(4) G(8) H(7) I(5) J(4) K(5) L(5)
The following is a sequence of waveform maps showing how this
alternative method is carried out: Starting Waveform Map:
TABLE-US-00010 A(0) B(0) C(0) D(0) E(0) F(0) G(0) H(0) I(0) J(0)
K(0) L(0)
Waveform Map after Pixel A is processed:
TABLE-US-00011 A(WG) B(+11) C(0) D(0) E(0) F(0) G(+8) H(+2) I(0)
J(0) K(0) L(0)
Waveform Map after Pixel B is processed:
TABLE-US-00012 A(WG) B(WG) C(+33) D(0) E(0) F(0) G(+22) H(+26)
I(+5) J(0) K(0) L(0)
Waveform Map after Pixel C is processed:
TABLE-US-00013 A(WG) B(WG) C(KG) D(-27) E(0) F(0) G(+22) H(+14)
I(-14) J(-4) K(0) L(0)
The Starting Waveform Map is the initial state of the waveform map
in which each pixel shows a cumulative error value of 0.
The error diffusion also progresses in the waveform map from left
to right and top to bottom, the process is performed from pixel A
to pixel L, one pixel at a time.
For pixel A, since the initial cumulative error value is 0, either
waveform WG or waveform KG may be chosen. If waveform WG is
selected, the shift error based on the grey level variation chart
in Example 2 would be +26 (234-208) for grey level 10 (which is the
target grey level for pixel A).
Then based on the Floyd-Steinberg algorithm, this shift error value
of +26 is diffused to the neighboring pixels: +11 (+26.times. 7/16)
to pixel B, +8 (+26.times. 5/16) to pixel G and +2 (+26.times.
1/16) to pixel H, as shown in the Waveform Map, after Pixel A is
processed.
The processing of pixel B, however, is different from that shown in
Example 3. In this case, both the WG and KG waveforms are
considered. For the WG waveform to drive pixel B to the target grey
level 5, the shift error would be +65 (210-145) and for the KG
waveform to drive pixel B to the target grey level 5, the shift
error would be -92 (42-134), based on the grey level variation
chart in Example 2. Each of the shift errors is then added to the
existing cumulative error value of +11 from processing of previous
pixel(s) (i.e., pixel A in this case). The sums of "the shift error
value and the cumulative error value" are then +76 (+65+11) and -81
(-92+11) for the WG and KG waveforms respectively. According to the
alternative method, waveform WG would be selected because its sum
of "the shift error value and the existing cumulative error value"
has a smaller absolute value (76 vs. 81).
The cumulative error of +76 is then diffused to neighboring pixels
(C, G, H & I) based on the Floyd-Steinberg algorithm. The
result is shown in the Waveform Map, after Pixel B is
processed.
It is noted that the error value diffused from pixel B must be
mathematically added to the existing cumulative error value from
processing of previous pixels. For example, pixel G already has an
existing cumulative error value of +8 and now an error value of +14
(+76.times. 3/16) is diffused to this pixel, resulting in a
cumulative error value of +22 in the Waveform Map, after Pixel B is
processed.
For pixel C, its target grey level is 4. If the WG waveform is
chosen, it would have a shift error of +79 ((197-118) and if the KG
waveform is chosen, it would then have a shift error of -95
(25-120). The sums of "the shift error value and the existing
cumulative error", in this case, would be +112 (79+33) and -62
(-95+33) for the WG and KG waveforms respectively. Since the sum
from the KG waveform has a smaller absolute value (62 vs. 112), it
is selected for pixel C.
The cumulative error of -62 is then diffused to the neighboring
pixels (D, H, I & J) based on the Floyd-Steinberg algorithm.
The result is shown in the Waveform Map, after Pixel C is
processed.
This process continues (from left to right and top to bottom) until
the waveform map is complete to show which pixel is driven by which
waveform. Final Waveform Map:
TABLE-US-00014 A(WG) B(WG) C(KG) D(G) E(WG) F(KG) G(KG) H(WG) I(WG)
J(KG) K(WG) L(WG)
This alternative method is useful because it may further reduce the
local errors by selecting a waveform which would generate a smaller
absolute error value.
It is noted that while the Floyd-Steinberg algorithm is used in
this example, other error diffusion algorithms may also be
similarly applied.
Example 6
Block Diagram of Hardware for Example 5
A block diagram in FIG. 8 illustrates the method demonstrated in
Example 5. As shown, the sum of the cumulative error for a pixel in
the waveform map (80) and the shift error shift values for both
waveforms (the first waveform 81a and the second waveform 81b) from
the grey level variation chart (84) based on the desired color (82)
would determine which waveform is selected. Both the selected
waveform and the desired color (82) of the pixel are input into the
look-up table module (83). The data thus generated from the look-up
table module are then output to the display panel.
In the meantime, the sum of the shift error value from a grey level
variation chart (84) based on the selected waveform and the desired
color (82), and the cumulative error for the pixel in the waveform
map (80) undergoes the process of error diffusion (85). The error
value diffused to each of the neighboring pixels is then
mathematically added to the cumulative error value for that
neighboring pixel, resulting in an updated waveform map. The
process as described continues.
Example 7
Another Example of Mono-Polar Waveforms
FIG. 5 shows alternative mono-polar driving waveforms which would
be suitable for the present invention. As shown, there are two
driving waveforms, WKG and KWG. When applying the two waveforms,
the WKG waveform drive pixels in the first group to the full white
state, then to the full black state and finally to a desired color
state. The KWG waveform, on the other hand, drives pixels in the
second group to the full black state, then to the full white state
and finally to a desired color state.
The WKG waveform has a tendency to cause the grey levels to shift
towards the darker color, due to speed decay caused by the medium
degradation. The KWG waveform has a tendency to cause the grey
levels to shift towards the lighter color, due to speed decay.
When utilizing this set of waveforms, one of the driving methods of
the present invention may be summarized as follows:
a) selecting the WKG or KWG waveform to drive a pixel to a desired
color, based on a cumulative error value resulted from processing
of previous pixels, wherein the WKG waveform tends to shift the
grey level color states between the black and white colors states
towards the black color after degradation, and the KWG waveform
tends to shift the grey level color states between the black and
white color states towards the white color after degradation;
b) determining a shift error value from a grey level variation
chart based on the waveform selected in (a) above and the desired
color of the pixel;
c) adding the shift error value to the cumulative error value of
the pixel; and
d) performing error diffusion.
The alternative driving method may be summarized as
a) determining shift error values for both the WKG and KWG
waveforms from a grey level variation chart based on the desired
color of a pixel, wherein the WKG waveform tends to shift the grey
level color states between the black and white colors states
towards the black color after degradation, and the KWG waveform
tends to shift the grey level color states between the black and
white color states towards the white color after degradation;
b) adding each of the shift error values to the cumulative error
value of the pixel;
c) selecting the WKG or KWG waveform whose sum of the shift error
value and the cumulative error value has a smaller absolute
value;
d) determining a shift error value from a grey level variation
chart based on the waveform selected in (c) above and the desired
color of the pixel;
e) adding the shift error value to the cumulative error value of
the pixel; and
f) performing error diffusion.
Example 8
Bi-Polar Waveforms
For bi-polar applications, it is possible to update areas from a
first color to a second color and also areas from the second color
to the first color, at the same time. The bi-polar approach
requires no modulation of the common electrode and the driving from
one image to another image may be accomplished, as stated, in the
same driving phase. For bi-polar driving, no waveform is applied to
the common electrode.
The two bi-polar waveforms WG and KG are shown in FIG. 6a and FIG.
6b, respectively. The bi-polar driving method has only two phases.
In addition, as the common electrode in a bi-polar driving method
is maintained at ground, the WG and KG waveforms can run
independently without being restricted to the shared common
electrode.
The methods of the present invention can be applied to the timing
controller (T-con) to process the waveform map in real time.
Therefore, the actual users do not have to perform any tasks to
achieve the desired results.
While the present invention has been described with reference to
the specific embodiments thereof, it should be understood by those
skilled in the art that various changes may be made and equivalents
may be substituted without departing from the true spirit and scope
of the invention. In addition, many modifications may be made to
adapt a particular situation, materials, compositions, processes,
process step or steps, to the objective, spirit and scope of the
present invention. All such modifications are intended to be within
the scope of the claims appended hereto.
* * * * *